<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        .box,.box2{
            width: 500px;
            height: 350px;
            border: 5px solid green;
            margin: 100px auto;
      
        }
        .box div{
            width: 100px;
            height: 100px;
            line-height: 100px;
            background-color: red;
            color: white;
            text-align: center;
            margin: 2px;
        }
        .box div:first-child{
            float: left;
        }
    
    </style>
</head>
<body>
    <div class="box">
       <div>1</div>
       <div>2</div>
       <div>3</div>
    </div>
    
</body>
</html>